The present invention relates to a multiple file system.
Multiple file systems have been used to achieve computer systems of higher availability and higher reliability. A well-known multiple file system comprises a first file unit and a second file unit such as a disk unit. The first and second file units are controlled so as to constantly hold the same files of data, each of which is accompanied by a revision number (e.g. date and time). The revision number is updated whenever data included in the corresponding file are updated. If a failure arises in the first file unit, only the files held in the second file unit can be updated and the revision numbers of the updated files are accordingly updated. Upon recovery from the failure in the first file unit, file-copying processing then takes place, so that the revision numbers of each pair of corresponding files in the two file units are compared and, for each pair of files whose revision numbers are found different, all the data of one file of the pair having the older revision number is replaced with those of the other file having the newer revision number.
The conventional multiple file system, however, has the following disadvantage. If a write request for updating a file (A) arises after a first failure occurs in the first file unit, data (B) of the file A in the second file unit are updated into data (C) without data (B) of the file (A) in the first file unit being updated. Next, when the first file unit recovers from the first failure after occurrence of a second failure in the second file unit, the above-mentioned file-copying processing cannot take place because the second file unit is in trouble at this time. If another write request for updating the file (A) arises after that, data (D) of the file (A) (whose data (B) have not updated) held in the first file unit are updated. Upon recovery of the second file unit from the second failure after that, all the data of the file (A) in the second file unit is replaced with all the data of the file (A) in the first file unit, and consequently data (C) are lost.
An object of the invention is, therefore, to provide a multiple file system free from the above-mentioned disadvantage.